1. Field of the Invention
The present invention relates to a microscope apparatus such as an optical microscope for optically observing an observation sample of a movable stage by using an objective lens, and relates in particular to a microscope apparatus having an autofocus (AF) function that allows automatic adjustment of a point of focus of the observation sample.
2. Description of the Related Art
Conventionally, microscopes that allow observation of fine samples and record the observed image as a video image have been in widespread use, from researches in the field of biology to inspection processes in the industry. Such microscopes have an AF function that allows automatic adjustment of a point of focus of an observation sample.
Normally, the AF is performed by using infrared light (AF light), and observation of a sample is performed by using visible light (observation light). For that reason, there may be variations in autofocus detection position due to the difference in wavelength of these two types of light and due to chromatic aberration for each of the objective lenses when plural objective lenses are used. As a means to compensate these variations, as described in Japanese Patent No. 4097761, for example, a technology of canceling out the chromatic aberration between the observation light and the AF light by arranging an offset lens for chromatic aberration in an in-focus detection optical system, setting an offset value from an in-focus point for each of the objective lenses, and adjusting the position of the offset lens for chromatic aberration has been developed.
The AF light is reflected at an interface between a cover glass with a relatively high reflectance and an observation sample or a surface of the cover glass, and the reflected light is detected. On the other hand, the observation light has to irradiate the observation sample itself, with a sample in liquid for example, and the reflected light needs to be detected. Therefore the lengths of optical paths of these two types of light are different. In order to correct these, a technology for matching the position of the observation sample to the focus of the objective lens by arranging a focusing offset lens on an optical path common to an optical system of the AF light and an optical system of illumination light or an optical path of either one of the optical systems, and for adjusting a position to form an image of the AF light by the focusing offset lens, as described in Japanese Patent Application Publication No. 2004-70276, for example, has been disclosed.
In such an AF function, a region of a photodiode that detects light is divided into two regions (A region and B region) at an optical axis of the reflected light as a center, and two sensors (sensors A and B) detect the light intensity of the respective regions as detection signals. A value obtained by dividing the difference between the signals (A−B) by the sum of the signals (A+B) is calculated ((A−B)/(A+B)) as an evaluation function value (EF value), and focus measurement is performed by using the EF value. In other words, the difference between the objective lens and the observation sample is changed relative to each other and a position at which the EF value becomes 0 is determined to be an in-focus position.
However, in the above-described detection signal of the light intensity, the beam diameters of the AF light at a pupil position of the objective lens are different depending on the position of the offset lens for chromatic aberration or of the focusing offset lens, and consequently, NA of light emitted from the objective lenses becomes different. As a result, variations are generated in the slope of the EF value in a state close to in focus. As the slope of the EF value becomes steeper, a range of an in-focus threshold at the time of focus measurement becomes narrower. In addition, when the slope of the EF value becomes steep because of the position of the offset lens for chromatic aberration or of the focusing offset lens, it becomes difficult for the tracking of in-focus positions to settle due to an influence of the ambient temperature or vibration.